1. Field of the Invention
This invention relates to methods and systems for locally connecting microstructures and devices formed thereby.
2. Background Art
Microelectromechanical sensors and actuators, as well as circuits for interfacing, control, and communications, have matured significantly in recent years. In order for this technology to have a pervasive impact, however, the diverse electronic and microelectromechanical devices must be integrated, interconnected, and packaged to build viable Microsystems. These Microsystems will be widely used in environmental and biological monitoring, implantable medical devices, and miniature robotics. Many applications require the Microsystems to be extremely small in order to minimize intrusion on the measurement environment. Certain applications also demand the ability to connect and disconnect delicate microstructures in the field, where complex assembly equipment is unavailable. For instance, a biologist may need to swap damaged off-board sensors in an insect-mounted microsystem without removing the system and returning to the laboratory.
FIG. 1 shows a hybrid microsystem, generally indicated at 10, developed for monitoring/controlling insects for biomimetic studies leading toward the development of legged robots and “biobots.” The microsystem 10 includes bare-die components 12, a custom printed circuit board 14 and commercial connectors 16. The system 10 contains signal-conditioning circuits for amplifying and multiplexing sensor data from off-board neural probes and mechanical sensors. Although this system 10 is sufficient for use with some insects, its size is prohibitively large for many others.
Approaches to shrinking such systems include monolithic circuit integration, reducing interconnect area by using silicon-based multi-chip modules, and implementing advanced packaging ideas such as folding platforms or three-dimensional assemblies. An important need in further reducing the size of such Microsystems is improved technology for connecting sensors to the platform using minimal area. The connectors 16 around the perimeter of the platform shown in FIG. 1 enable sensors to be connected and disconnected manually; however, they consume an inordinate amount of board area. New developments in connector technology are necessary in order to realize smaller systems.
Solder has long been used in electronic assembly to mechanically and electrically secure packaged components to printed wiring boards. Eutectic metal-alloy solders are designed to have low temperature melting points in order to minimize the temperatures to which components are exposed. Components can be easily removed by withdrawing them while heating the bonding areas. Thus, the use of solder-based interconnects has a number of attractive features. However, traditional soldering methods pose several problems. Sensors and other microstructures are increasingly delicate and can be easily damaged or destroyed, both thermally and mechanically. Use of soldering irons or hot-air tools that are common in the macro-world is not always possible in the micro-domain. For example, using insect-mounted “backpack” Microsystems for gait studies requires connections to leg strain gauges and EMG wires in the field; heating the platform to bonding temperatures with a soldering iron or reflow oven is precluded. For this and many other applications, a different approach to solder connections is necessary.
Many of the problems associated with external heating can be avoided by building heaters into the platform to heat the pad areas only. Chen and Lin, as described in their article entitled “Localized bonding with PSG or indium solder as intermediate layer,” MEMS 1999, pp. 285-289, have exploited this concept for silicon-to-glass bonding using localized heating of indium solder.
If solder bonding is to be useful for microconnections, the technology must be scalable to sub-millimeter dimensions. Solder has rarely been used for lead-pitches less than 1 mm, and many applications have much less stringent size requirements.
Recently, solder balls for flip-chip bonding and chip-scale packages have been used commercially at pitches as small as 0.75 mm (Intel Corporation, Intel Flash Memory Chip Scale Package User's Guide, 1999).
Harsh and Lee have used surface-area minimization of solder during melting for MEMS self-assembly, as described in their article, “Study of micro-scale limits of solder self-assembly for MEMS,” Electronics Components & Technology Conference, 2000 Proceedings, pp. 1690-1695. They report experiments and simulations indicating that the physical behavior of solder should remain consistent with scaling, even to sub-micron dimensions. This work suggests that solder should behave predictably on the scale 25 μm to 500 μm—the regime of interest for microconnections to components such as MEMS sensors. Also, solder's tendency to coagulate and draw structures together is potentially beneficial to microconnections.
The variation of fringing capacitance induced by bringing an object into close proximity of multiple metal pads has been exploited in several ways. Franklin et al. used this concept for an electronic wall stud sensor, as described in U.S. Pat. No. 4,099,118.
Tartagni et al., as described in “A 390 dpi Live Fingerprint Imager Based on Feedback Capacitive Sensing Scheme,” ISSCC 1997, pp. 200-201, have used a technology for live fingerprint imaging based on a feedback capacitive sensing scheme. Here, the ridges and valleys of a human fingerprint are detected by an array of divided pads. The capacitance between neighboring pads varies according to whether skin (a ridge) or air (a valley) overlays the plates.